The present invention relates to a method for adjusting a lookup table comprising preset on-time values for an injector of a cylinder in a combustion engine of a motor vehicle linked to fuel injection quantity values, where the fuel injection quantity values represent different desired quantities of fuel to be injected into the cylinder and the on-time values represent the associated on-times for the injector. The invention also relates to a system for controlling an injector of a cylinder in a combustion engine of a motor vehicle. Furthermore, the invention relates to a computer program comprising computer program code for implementing the method, a computer program product comprising a data storage medium readable by an electronic control unit and having said computer program stored thereon, and an electronic control unit.
In this description and the subsequent claims, the term “on-time” refers to the opening time of an injector, i.e. the duration of the time period during which the injector is kept open in order to inject fuel into the associated cylinder in connection with a single working stroke. The quantity of fuel injected into a cylinder in connection with a working stroke depends on the length of this on-time and the pressure of the fuel supplied to the injector.
A conventional combustion engine for a motor vehicle, such as for instance a car, lorry, towing vehicle or bus, is provided with injectors for injecting desired quantities of fuel at desired moments into the cylinders of the engine. The injectors are controlled by means of an engine control unit, which controls the on-times for each individual injector so as to thereby control the quantity of fuel to be injected into the cylinder associated with the injector. It is a common practice to establish suitable on-times for the injectors with the aid of a lookup table comprising preset on-time values for the injectors linked to fuel injection quantity values, where the fuel injection quantity values represent different desired quantities of fuel to be injected into each cylinder of the engine and the on-time values represent the associated on-times for each injector. For a common rail fuel injection system, the on-times of such a lookup table are linked both to such fuel injection quantity values and to common rail pressure values representing different fuel pressures in the common rail. The engine control unit determines the present fuel injection quantity value, i.e. the desired quantity of fuel to be injected by an injector at the present working stroke, with the aid of a regulator in dependence on the present operating conditions of the vehicle. Based on the present fuel injection quantity value and, in case of a common rail fuel injection system, the present common rail pressure value, the engine control unit will establish a present on-time for an injector with the aid of said lookup table. This on-time is expected to result in the injection of the desired fuel quantity, i.e. a fuel quantity corresponding to said present fuel injection quantity value. However, in course of time, the characteristics of the injectors may change in such a manner that the preset on-time values of the lookup table will not give the expected fuel injection quantities. Such deviations between expected and actual fuel injection quantities may result in uneven idling and lost pilot or post injections.